On the back of an envelope?

Seriously: with a pen, a scrap of paper, and a calculator.

If I want to keep the calculation I'll copy it to a plain text document or to a spreadsheet, or copy it to a notebook.

Also, if you have a browser open, you can just type equations into the console (using JS syntax):

  > 3 + 4 / ( 5 * 0.9 + Math.PI) + Math.pow(2, 4)
  < 19.52345108949519

Edit: or just open Terminal, type 'python', and use in interactive mode:

  >>> import math
  >>> 3 + 4 / ( 5 * 0.9 + math.pi) + math.pow(2, 4)
  19.52345108949519